During exercise, in order for the working muscles to be receiving adequate amounts of nutrients and oxygen, the heart rate needs to be increased such that waste products which include the lactic and carbon dioxide are being removed. Anticipatory rise in heart rate occurs before even starting to exercise due to the release of adrenaline in the sympathetic nervous system.

Once exercise has begun, there is an increase in carbon dioxide and lactic acid in the body which is detected by chemoreceptors. The chemoreceptors trigger the sympathetic nervous system so as to increase the release of adrenaline, which will further increase heart rate. As exercise continues, the body core temperature rises, which will also help to increase the heart rate since it increases the speed of the conduction of nerve impulse across the heart.

The body’s gas transport system consists of the heart, lungs and hemoglobin, which connects the atmosphere and its supply of O2 with tissue, while simultaneously providing the mechanism for the elimination of the metabolic end-product, CO2 back into the atmosphere. It is important that the transport of these respiratory gases must be in accordance with metabolic need. This is particularly evident during the physiologic stress of isotonic exercise, when the O2 requirements and CO2 production of skeletal muscle are increased (Cardiol, Am J, 1985).

Oxygen which is being carried in the blood is either dissolved in the plasma or combined with hemoglobin. However, as oxygen is not readily soluble in fluids, only about 3 ml oxygen can be carried per liter of plasma. Nevertheless, this limited amount of oxygen that is transported in plasma does contribute to the partial pressure of oxygen in blood and other body fluids. This helps in playing a role in the mechanisms which control breathing and also in the diffusion of oxygen into the alveolar blood and into the cells of body tissues.

There are some similarities to the oxygen transport in the way in which carbon dioxide is being removed from the system. However, the huge amount of carbon dioxide is removed by a more complicated process. The carbon dioxide is transported out of the cell by diffusion and subsequently transported to the lungs, after carbon dioxide is formed in the cell.

Similar to oxygen, only a limited amount of carbon dioxide of about 5% of that produced during metabolism is being carried in the plasma and likewise, this limited amount of carbon dioxide also helps to contribute to establishing the partial pressure of carbon dioxide in the blood. Some small amount of carbon dioxide is also being transported via hemoglobin.

Although the above mentioned processes are important, the greatest amount of carbon dioxide removal (approximately 70%) results from a process that involves its combination with water and its delivery to the lungs in the form of bicarbonate. The first step in this reversible reaction is the combination of carbon dioxide in solution with water in the red blood cells to form carbonic acid. The reaction process is normally quite slow except for the impact of the enzyme carbonic anhydrase, which drastically speeds up this process. Once carbonic acid is formed, it is broken down to hydrogen ions and bicarbonate ions.

Since hemoglobin is a significant acid-base buffer, hydrogen ions will combine with hemoglobin and this process helps to maintain the pH level of the blood. The bicarbonate ions will in turn diffuse from the red blood cells out to the plasma while chloride ions diffuse into the blood cells to replace them (Baechle, Thomas R. and Roger W.Earle, 2000).

The oxygen dissociation curve is an S shaped curve as can be seen in Figure 3, which represents the percentage of hemoglobin (Hgb) fully saturated with O2 (y-axis) and the partial pressure of O2 (PO2, in mmHG) in the blood at normal physiologic conditions (x-axis). This is also the ease with which haemoglobin will release oxygen when exposed to tissues of different oxygen concentrations. The curve begins with a sharp rise as haemoglobin has a high affinity for oxygen, which means that for a small rise in the partial pressure of oxygen, haemoglobin will pick up and bind oxygen to it readily.

However, only a small drop in the partial pressure of oxygen will result in a large drop in the percentage saturation of haemoglobin. Hence, in exercising muscles, where there is a low partial pressure of oxygen, the haemoglobin will readily unload the oxygen for use by the tissues (Stafford-Brown, Jennifer et al., 2003).

The factors which affect the oxygen-hemoglobin dissociation curve, where high pH, low pCO2, low temperatures and the absence of disphosphoglycerate (DPG) will shift the curve to the left and reflect an increase in O2 affinity (left column). Contrarily, lower pH, higher pCO2, higher temperatures and the presence of DPG shift the curve to the right and reflect an decrease O2 affinity (right column) (Diallo, Alfa, 2006).

The complete saturation of haemoglobin at sea level is 98%, and there are many factors which influence the haemoglobin saturation as follows:

1. PO2 values
2. Decline in pH level from increasing lactate levels allows more oxygen to be unloaded and higher PO2 is needed to saturate the haemoglobin.
3. Increased blood temperature allows oxygen to unload more efficiently and higher PO2 is needed to saturate the haemoglobin.
4. Anemia reduces the blood's oxygen-carrying capacity(Kauth, Bill, 2005)

Training principles, specificity, overload, progressive, reversibility, variance.